Solar cells capable of converting sunray into electric power as energy sources in place of fossil fuel have been drawing attention. Presently, solar cells whose commercial use has partially been begun are solar cells using crystalline silicon substrates and thin film silicon solar cells. However, the former costs high for fabrication of the silicon substrates, and the latter requires various kinds of semiconductor gases and complicated apparatuses and the high production cost still remains as a problem. Therefore, it has been tried to lower the cost per electric power output by increasing the efficiency of the photoelectric conversion of both types of solar cells, but the above-mentioned problem has not been solved yet.
As a new type solar cell, Japanese Patent No. 2,664,194 discloses a wet type solar cell based on photo-induced electron transfer of a metal complex. The wet type solar cell comprises a photoelectric conversion element using a photoelectric conversion material and an electrolytic material and sandwiched between two glass substrate. The photoelectric conversion material has an absorption spectrum in the visible light region by adsorbing a photosensitizing dye. In the wet solar cell, when light is radiated to the photoelectric conversion layer, photoelectron is induced and the electron is transferred to an electrode through an external electric circuit. The electron transferred to the electrode is turned back to the photoelectric conversion layer by being conveyed through ion in an electrolytic substance. Electric energy is outputted by repeat of such electron transfer.
A technique of a low cost fabrication method of the wet type solar cell based on the above-mentioned operation principle is disclosed in Japanese Patent Application Laid-Open No. 2000-91609. To explain the outline of the technique, first, a glass substrate in which a transparent conductive film (electrode) is formed is made ready. Further, a platinum conductive film (electrode) and a colloidal titanium dioxide power generation layer are formed on another substrate so flexible to be rolled so as to obtain a layered body. At the time of or after the layered body formation, the power generation layer is impregnated with an electrolytic solution. According to this technique, a single unit organic solar cell can be obtained.
Also, PCT WO 97/16838 pamphlet discloses, as shown in FIG. 7, a dye-sensitized solar cell module comprising a plurality of dye-sensitized solar cells connected in series. Practically, each dye-sensitized solar cell has a structure composed by successively layering a titanium oxide layer, an insulating porous layer, and a counter electrode on a glass substrate on which a transparent conductive film (electrode) is patterned into a strip-like form. The conductive layer of one dye-sensitized solar cell is arranged so as to contact with the counter electrode of another neighboring dye-sensitized solar cell to connect both solar cells in series.
Also, P. M. Sommeling et al. disclose a dye-sensitized solar cell module having a structure of W type series connection as shown in FIG. 8 in Development Technology of Dye-Sensitized Solar Cell, edited by Shuji HAYASE, & Akira FUJISHIMA, issued by Gijutsu Kyoiku Shuppan Sha, p. 205-217 (2003).
Practically, titanium oxide layers and platinum layers are alternately formed on each of two glass substrates on which a transparent conductive electrode is patterned into a strip-like form: the substrates are layered each other in a manner the titanium oxide layer and the platinum layer are set face to face: an insulating adhesive such as a resin is put between the respective pairs of the titanium oxide layers and the platinum layers: and the layered glass substrates are stuck to each other by the insulating adhesive to obtain each dye-sensitized solar cell. And a dye-sensitized solar cell module is produced by connecting such dye-sensitized solar cell in series.
However, the basic structure of the dye-sensitized solar cell described in Japanese Patent No. 2,664,194 is composed by injecting an electrolytic solution between two glass substrates. Accordingly, even if a solar cell with a small surface area can be produced on an experimental basis, it is difficult to apply the method for producing a solar cell with as a large surface area as 1 m-square. With respect to such a solar cell, if the surface area of one solar cell (unit cell) is enlarged, the generated current is increased proportionally to the surface area. However, the voltage is extremely decreased in the direction of the electric current flow in the transparent conductive film to be used as an electrode part and consequently, the inner series electric resistance of the solar cell is increased. As a result, there occurs a problem that fill factor (FF) in electric current-voltage property at the time of photoelectric conversion is decreased and accordingly the photoelectric conversion efficiency becomes low.
To solve the above-mentioned problem, it seems to be possible to form an integrated structure by bringing a first conductive layer of a rectangular unit cell employed for a module of amorphous silicon solar cells each comprising an amorphous silicon layer sandwiched between a first conductive layer and a second conductive layer into contact with a second conductive layer of a neighboring unit cell. However, in this structure, it is required to keep a certain gap so as to keep neighboring photoelectric conversion layers from one another. Generally, the conversion efficiency of an integrated solar cell module means the electric power generation efficiency per surface area of the module. Therefore, if the surface area of the gap is wide, the light comes into the gap does not contribute to the power generation and even if the conversion efficiency of unit cells composing a module is high, the conversion efficiency as a module becomes low. Accordingly, it is required to develop a production method of a module in which the gap between neighboring unit cells is narrowed.
In general, in an amorphous silicon type solar cell, integrated patterning is carried out by scribing with laser, but this technique application to a dye-sensitized solar cell is difficult. It is because a large quantity of dye is to be adsorbed in a photoelectric conversion layer of the dye-sensitized solar cell and therefore the layer is made of a porous material. The part of the porous material in which a fine pattern is formed with laser as described is inferior in the strength: and thus the fine pattern formation becomes impossible.
Further, there is another problem that use of laser increases the production cost.
To solve the above-mentioned problems, a porous photoelectric conversion layer is formed by a screen printing method in a solar cell described in PCT WO 97/16838 pamphlet as shown in FIG. 7. However, pattern formation is carried out by laser or air jet after formation of a porous photoelectric conversion layer and similar problems as described above are caused because of the similar reasons as described above. In FIG. 7, the reference numeral 41 denotes a transparent substrate; 42 denotes a transparent conductive film; 43 denotes a porous titanium oxide layer; 44 denotes an intermediate porous layer; 45 denotes a counter electrode; 46 denotes an insulating layer; 47 denotes a top cover for air-tightly sealing an electric insulation liquid; and 48 and 49 denote terminals.
Also, in a solar cell shown in FIG. 8, since a titanium oxide layer and a light transmitting counter electrode are formed in the plane direction of one glass substrate having a transparent conductive film, regardless of the front or rear face of a solar cell module, a light receiving face can be set, however with respect to a solar cell having the light receiving face in the counter electrode side, since light is absorbed by the electrolytic solution in the solar cell as compared with the solar cell in which the counter electrode side does not become light receiving face, the incident light quantity is decreased to result in a problem of decreased of the electric current. Accordingly, the solar cell module produced by connecting solar cells in series has a problem that the properties of the solar cells composing the module cannot be exhibited at the maximum. In FIG. 8, the reference numerals 31 and 32 respectively denote a transparent substrate; 301, 302, and 303 respectively denote a transparent conductive film; 311, 312, and 313 denote an electrolytic solution; 321, 322, and 323 denote a porous titanium oxide layer; 331, 332, and 333 denote a catalyst layer; and 341, 342, and 343 denote an insulating layer.